In recent years, image forming apparatus using an electrophotographic system, such as copying machines and printers, have rapidly trended toward formation of full-color images. In color printing by the electrophotographic system, even high-definition images including photographs are printed, so that there is a demand for printing high in resolution and good in color reproducibility. Therefore, high-quality color toners, which can meet requirements for such high printability, are required.
In image formation by the electrophotographic system, an electrostatic latent image formed on a photosensitive member is developed with a developer (may referred to as “toner” merely). The developer comprises, as a functional component, colored resin particles. The colored resin particles are roughly divided into a pulverized toner obtained by pulverizing a resin composition comprising a binder resin and a colorant, and a polymerized toner obtained by polymerizing a polymerizable monomer composition comprising a polymerizable monomer and a colorant in an aqueous dispersion medium.
According to the polymerization process, spherical colored resin particles (colored polymer particles) can be efficiently produced. In general, the spherical toner is suitable for use as a color toner because it is good in transferability and dot reproducibility. Therefore, the polymerized toner produced by the polymerization process has been about to be used mainly in the field of developers in recent years. A suspension polymerization process is representative of the polymerization process. Besides this process, a dispersion polymerization process and an emulsion polymerization process are also known.
A one-component developer is obtained by optionally adding an external additive to the colored polymer particles obtained by the polymerization process, and a two-component developer is obtained by mixing carrier particles with the particles. When magnetic powder is contained in a polymerizable monomer to polymerize the monomer, a polymerized toner having magnetism is obtained. Developers (toners) containing the polymerized toner as a functional component are classified into 4 kinds of a magnetic one-component developer, a non-magnetic one-component developer, a magnetic two-component developer and a non-magnetic two-component developer according to the presence of a carrier and magnetic powder.
As colorants, are used black colorants such as carbon black, and besides colorants for color toners, such as yellow colorants, magenta colorants and cyan colorants. Most of these colorants are pigments or dyes substantially insoluble in polymerizable monomers.
In order to form a high-definition image using the polymerized toner, it is required to finely and uniformly disperse the colorant in the polymerized toner. Therefore, in a process for the production of the polymerized toner, it is first necessary to finely disperse the colorant in a polymerizable monomer. There have heretofore been proposed methods using various kinds of media dispersing machines as methods for dispersing the colorant in the polymerizable monomer.
For example, Japanese Patent Application Laid-Open No. 6-75429 has proposed a production process of a polymerized toner making use of a media dispersing machine shown in FIG. 5 in the step of dispersing a colorant in a polymerizable monomer.
The media dispersing machine shown in FIG. 5 has a structure that a plurality of agitator discs (i.e., “rotors”) 507 fitted on a driving shaft 510 are arranged within a cylindrical casing (i.e., “stator”) 501 having a liquid supply port 502, and a great number of media particles 508 are contained in an internal space. A jacket having a cooling medium inlet 504 and a cooling medium outlet 505 is arranged on an external peripheral surface of the cylindrical casing 501 in such a manner that the internal temperature of the casing can be controlled by a cooling medium.
When a liquid mixture (slurry) containing at least a polymerizable monomer and a colorant is continuously introduced into the casing from the liquid supply port 502 while rotating the agitator discs 507 by rotation of the driving shaft 510, strong shearing force is applied to the media particles and the liquid mixture, and the colorant is pulverized in the liquid mixture and finely dispersed therein. The liquid mixture (i.e., “dispersion liquid”) with the colorant finely dispersed is separated from the media particles by a media-separating gap separator 509 and carried to the outside from a liquid discharge port 503 provided at an upper portion of the casing 501.
Since the media dispersing machine shown in FIG. 5 is wide in the internal space, a packing phenomenon that the media particles filled are forcedly pressed against the inner surface of the casing by the centrifugal force of the agitator discs is markedly caused when the peripheral velocity of the tip of the agitator disc is increased for raising the ability to pulverize the colorant, so that the layer of the media particles filled becomes uneven. As a result, the so-called short pass that the flow of the liquid mixture is liable to pass through a thin portion of the media particle layer has occurred to involve a problem that the efficiency of pulverization•dispersion and evenness are lowered.
In addition to the above problem, this media dispersing machine has involved a problem that since the media particles are unevenly distributed to the media separation gap separator portion, and the efficiency of dispersion is lowered, it takes a long time for treatment, and moreover a production efficiency is greatly lowered because the treatment speed is lowered by the pressure increase within the apparatus. In fact, Examples of Japanese Patent Application Laid-Open No. 6-75429 only show experimental examples where the media dispersing machine is used to conduct dispersion with the peripheral velocity of the agitator (stator) agitating the media controlled to a fixed peripheral velocity within a range of 3 to 30 m/s. According to such a method, the degree of dispersion of the colorant becomes insufficient, or the treatment time becomes long to greatly lower a production efficiency.
U.S. Pat. No. 6,309,788 has proposed a method for dispersing a colorant in a polymerizable monomer by using a media particle agitating type wet dispersing machine, in the internal chamber of which a plurality of media particles are contained. Even when this method is used, however, there have been problems that the degree of dispersion of the colorant becomes insufficient, and that the media particles are unevenly distributed on to a slit of a separator to raise the internal pressure of the apparatus, and so a production efficiency is lowered.
Japanese Patent Application Laid-Open No. 2000-89508 discloses, as a means for dispersing a colorant in a polymerizable monomer, a method in which the colorant is pre-dispersed in the polymerizable monomer for 10 to 60 minutes under high-speed agitation at a peripheral velocity of 1.5 to 12 m/s, and dispersion is then conducted by a batch type dispersion machine using media. According to this method, however, sufficient dispersion cannot be achieved, or it takes a too long time to reach a sufficiently dispersed level.
U.S. Pat. No. 6,534,233 has proposed a method for dispersing a colorant by means of a media dispersing machine under conditions that the tip speed (peripheral velocity) of an agitator (rotor) is 3 to 20 m/s, and a residence time is 0.03 to 0.5 hour. Even by this method, a necessary dispersed level cannot be attained in the case of a color toner of which a particularly high dispersed level is required, or it takes a very long time to reach the dispersed level, so that productivity is deteriorated.